In the sorting of lumber according to its bending stiffness, the most commonly used machines and processes for high speed production facilities involve bending of boards as they are tested in a machine especially constructed for this purpose. Such machines use a series of rollers that bend each board in opposite directions as it passes longitudinally through the machine. Background information on such testing equipment and processes can be found in U.S. Pat. No. 3,196,672 (Keller), which is hereby incorporated into this disclosure by reference.
The machine described in the Keller patent disclosure causes substantially constant bending deflection of boards by a measuring transducer assembly that detects the force applied to each board by a load point on the transducer assembly. For the lumber testing machine example, the load point consists of a transverse line contact between a force roller and each board as it is bent. The force roller is part of the detecting transducer assembly. At high machine speeds, constant lumber deflection and fixed position of the transducer assembly is vital because they substantially remove the inertias of the transducer assembly from affecting the bending measurement.
The transducer assembly is located relative to the machine frame at two longitudinal reference points along the length of the transducer assembly. At one reference point two roller bearings on a common transverse bearing reference axis fix the transducer assembly in translation to the machine frame. At the other reference point, the transducer assembly rests against an electronic load cell used to measure force applied by the transducer assembly to the load cell. By this means, the force applied by the load point force roller to the lumber can be detected.
These reference points at both locations along the length of the transducer assembly are intended to fix the transducer assembly in translation so that the load point on the transducer assembly remains substantially constant relative to the machine frame. There is, however, a minute compliance required at the load cell reference point for a measurement of force to be made. For load cells used in the commercial implementation of the Keller patent disclosure, this compliance is about 33.3.times.10.sup.9 feet/pound (2.28.times.10.sup.9 meter/newton). Roller bearings are conventionally used in these machines at the bearing reference axis, so that as load cell compression occurs there will be no torsional restraint of the transducer assembly relative to the machine frame. If the bearings are properly adjusted with a small preload, they will allow slight angular movement of the transducer assembly about the bearing reference axis and prevent radial motion or axial or longitudinal translation between the transducer assembly and machine frame at that point.
Unfortunately, roller bearings operating in this mode are subject to brinelling failure (actually known as "false brinelling") caused by vibration with bearings stationary. This can be predicted. And, it is borne out in practice, as observed by the failure mode of these bearings. Bearings exhibiting false brinelling wear patterns will loosen as they wear, thereby allowing translational motion in their support of the transducer assembly which leads to accuracy problems in the load measurement. Further, if the bearings are tightened by adjustment to remove the translational freedom, the wear patterns serve as detent positions in the bearings and cause nonlinear restrictions in rotation of the transducer assembly about the bearing reference axis, and this also reduces accuracy. Another problem with the bearings now used has been the difficulty in rigidly attaching their inner races to the transducer assembly; any lack of translational rigidity at this point leads to accuracy and repeatability problems.
In the application of sorting lumber, one orientation of the transducer assembly requires a spring to maintain positive pressure of the transducer assembly against the load cell, thus countering the effect of gravity which, in the absence of the spring, would cause the assembly to pivot around the bearing reference axis and fall free of the load cell reference point. Constraining the spring so that it does the required job of preloading the transducer assembly against the load cell with negligible friction has been a continuing problem of this design. The sides of the spring can rub against the spring restraint, and the resulting friction leads to repeatability problems in calibrating the system's zero point.